1. Field of the Invention
The present invention relates to a method and apparatus for evaluating the surface roughness of an epitaxial growth layer, a method and apparatus for measuring the reflectance of an epitaxial growth layer, and a manufacturing method of a semiconductor device which are applicable to, for instance, a case where a base layer of a bipolar transistor that is required to operate at high speed is epitaxially grown.
2. Description of the Related Art
To increase the operation speed of bipolar transistors used in ICs, it is indispensable to form a thin, high-concentration base layer. However, in the conventional ion implantation technology, it is difficult to realize a base width of 40 nm or less because of channeling of an implantation impurity.
In recent years, a method of forming a base layer by using an epitaxial technique which is free of channeling has been studied extensively as one approach for solving the above problem. On the other hand, also in MOSFETs (metal-oxide-semiconductor field-effect transistors), the formation of epitaxial wafers and the selective epitaxial growth of source and drain portions that are advantageous in realizing a short channel have been studied with great expectations.
In the conventional, common silicon epitaxial growth, film formation is performed at as high a temperature as 1,000.degree. C.-1,200.degree. C. in an abundant high-purity hydrogen atmosphere. Therefore, the crystallinity of a resulting silicon epitaxial layer is rarely lowered unless there occurs a damaged layer on the substrate surface for silicon epitaxial growth or contamination of a heavy metal or the like. Further, a resulting silicon epitaxial layer has a flat crystal surface with very low surface roughness.
However, in the above-described case of forming a base layer of a bipolar transistor by epitaxial growth, it is necessary to obtain a steep impurity profile. To this end, the heat treatment temperature should be set as low as possible; silicon epitaxial growth is performed at as low a temperature as 550.degree. C.-900.degree. C.
To perform silicon growth at such a low temperature, film formation should be performed in an atmosphere having a very high level of cleanliness. In particular, epitaxial growth capable of providing good crystallinity cannot be performed unless partial pressures of water and oxygen inside a film forming apparatus are managed so as to be kept sufficiently low. These points are also applicable to the formation of epitaxial wafers and the selective epitaxial growth of source and drain portions for MOSFETs.
The reason why low-temperature silicon epitaxial growth should be performed in a high-cleanliness atmosphere as mentioned above will be described below with reference to FIGS. 1A-1C. Figs. 1A-1C are schematic sectional views of the main part showing a process of causing epitaxial growth on a silicon substrate 31.
When silicon epitaxial growth is performed at a low temperature, even if, for example, a clean surface is produced by a dilute hydrofluoric acid treatment or the like before the silicon substrate 31 is placed in an epitaxial growth apparatus, oxide films 32 are formed on the substrate 31 before occurrence of epitaxial growth or island-like oxide films 32 are formed in the midst of epitaxial growth as shown in FIG. 1B if the partial pressure of water or oxygen inside the apparatus is high. In such a case, stacking faults 34 tend to occur inside an epitaxial growth layer 33 as shown in FIG. 1C because silicon crystal growth is impaired. If the stacking faults 34 occur, the roughness of the surface of the epitaxial growth layer 33 (surface roughness) becomes high and its surface flatness is lowered.
G. Ghidini and F. W. Smith reported a relationship among the possibility of proper epitaxial growth, the growth temperature, and the partial pressure of water in an atmosphere (J. Electrochem. Soc., Vol. 131, p. 2924, 1984). FIG. 2 shows the above relationship that is described in this paper.
As indicated by an epitaxial growth possibility boundary straight line 35, FIG. 2 shows a tendency that oxidation is prone to occur and the surface roughness becomes high (a single crystal is not grown) unless film formation is performed at a water partial pressure that is lower than determined by the line 35 at a preset film forming temperature (horizontal axis). FIG. 2 also shows that oxidation is prone to occur unless the water partial pressure is set lower as the silicon epitaxial growth is performed at a lower temperature. Therefore, in the low-temperature silicon epitaxial growth, it is important to evaluate the crystallinity of an epitaxial growth layer by measuring its surface flatness (i.e., the degree of oxidation) and to feed back a result of the evaluation to the process conditions for the formation of the epitaxial growth layer.
As described above, the surface of a resulting silicon epitaxial layer is roughened if large defect layers exist inside the silicon epitaxial growth layer. The simplest method for evaluating such roughness is visual inspection with a light-gathering lamp in a darkroom. However, since this is based on sensory perception, it cannot evaluate roughness quantitatively.
Therefore, roughness evaluation is now commonly performed by using an atomic force microscope (AFM) which can detect asperity of an atomic level. However, the surface roughness measurement with an atomic force microscope is influenced by external vibration or noise. Therefore, it is necessary to provide an antivibration, soundproof structure at a location where a measuring apparatus is to be installed, which increasing the installation cost. Further, a measurement requires a high degree of skill and know-how of measurement techniques. There are additional drawbacks such as a long measurement time.